Error when scanning own participant QR code

[MatthewLM]

To reproduce:

1. Create a new ROAST group and click "Consent & Create Group"
2. Add another participant
3. Click "Share Your Public Key"
4. Take screenshot of QR code
5. Click "Add Member"
6. Try scanning own QR code
7. The wallet will close and an error will be shown.
8. Going back onto the wallet, it can be seen that the existing participant is removed.